a. Field of the Invention
The present invention relates to regulation of voltage from one or more electrical power substation transformers, and in particular, to control of voltage regulating tapchangers on substation transformers.
b. Problems in the Art
Electrical power generated in utility power plants has to be stepped down in voltage for residential and commercial use. Electrical power is most efficiently supplied to diverse locations by sending high voltage of many thousands of volts through supply transmission lines.
Electrical distribution substations are placed in scattered locations and contain transformers necessary to reduce transmission grid voltage (generally several thousands to tens of thousands of volts) to standard residential and commercial levels. The voltage also needs to be rather closely regulated to insure uniformity and reliability in light of the many and varied electrical loads which exist from moment to moment in residential and commercial uses.
Conventionally in the United States, residential voltage levels are around 110 VAC. Commercial voltage is usually around 220 VAC. Government standards require these voltages be maintained within approximately plus or minus 5 percent of these standards at all times.
It must be appreciated that conversion of what will be called the very high transmission or supply voltage to the substantially lower distribution voltage, with such precise regulation, depends not only on accurate transformers and associated equipment, it also depends on the varying conditions that can exist from moment to moment in electrical power distribution systems. For example, at times residential electrical power use can collectively increase in amount so that the collective load on the distribution lines causes the collective voltage to drop below acceptable limits. A well known example of such a situation is during hot weather periods where widespread and long usage of air conditioning can cause peak loading on a distribution system.
Another factor that comes into account is a fluctuation in supply voltage. This can be caused by a variety of factors at the power utility itself. It can result in a fluctuation in supply voltage to the substation transformers, which in turn can affect distribution voltage. Other factors and situations come into play which can affect accurate regulation of distribution voltage.
The regulation of output voltage from power transformers has been a requirement since the beginning of the electrical age. One well-known in the art method is to alter the position at which an output from one winding of the transformer is taken. The ratio of turns of that winding (usually the "secondary winding"), as compared to the other winding (usually called the "primary winding") can be altered, in turn altering the voltage "transformation" between primary and secondary windings.
A conventional structure to allow such alterations is to provide a plurality of terminals or "taps" along the winding. A translatable terminal is moveable with respect to the taps allowing selection of a desired tap.
Mechanisms to accomplish this function are generally called "tapchangers". As load on the distribution line increases, more supply voltage is needed to be "transformed" or presented by the transformer on the distribution side. By monitoring distribution line voltage, when a drop occurs, the tapchanger can be instructed to move to a tap that will send more voltage down the distribution line, to keep the voltage within the required range. Several generations of tapchanger controls have been developed to achieve this end.
The first voltage regulation system of this type was manually controlled by an operator who, through mechanical cranks or later electrical switches, could use his/her judgment as to when to operate tapchangers to change the distribution voltage to the customers of the utility.
The Ponstingl U.S. Pat. No. 3,423,657 is typical of electromechanical methods of control tapchangers. Its method uses older style tapchanger controls and hardware wiring to determine, through very limited intelligence, how to properly operate a furnace transformer. While the device does control a tapchanger to a manually selected control point, the device will not automatically allow for changing loads or operate multiple transformers. This system has severe limitations in utility substation applications.
The Pinney U.S. Pat. No. 2,777,369 describes a method of paralleling three transformers using a series of electromechanical controls and motor driven cam switches. The system sets up a master control and several follower controls. By monitoring circulating current, the combination of controls allows the paralleling of transformers. This design makes no calculation or use of reactive power. The electrical system is not monitored for switching conditions. The Pinney patent requires a control apparatus for each transformer. The controls do not provide alarming or lock-out functions. These factors represent deficiencies when applied to efficient operation of tapchangers in utility substations. A significant problem encountered when connecting substation transformers in parallel is that differences in impedances between transformers (however slight) can result in one supplying more load. This loading imbalance can also occur if one transformer supplies more load from the buses which interconnect the elements of the system. This is a significant reason why such transformers must be monitored and regulated.
Typical of many present conventional tapchanger controls is the L. E. Conner control of U.S. Pat. No. 3,252,078. This control uses various electronic setpoints to monitor voltage and make tapchanger adjustments. The control directly monitors voltage and current to arrive at a setpoint voltage and adjust the operating point according to load. However, the control does not provide any paralleling options. It does not perform self diagnostics or high/low voltage limit checking. The control cannot operate more than one tapchanger nor provide alarming for erroneous sensors or malfunctions.
Newer technology is utilized in the Jindrick invention of U.S. Pat. No. 4,419,619. The Jindrick control uses a microprocessor to scan the voltage and current of a transformer in order to regulate the voltage. The invention uses a dedicated control and software stored in read only memory to calculate and derive the voltage setpoint and bias it according to load. However, the control system parameters are varied by control dials which lose accuracy over time.
Patents such as Jindrick show that attempts continue to be made to improve the way voltage regulating tapchangers can be controlled at transformers for substations. While Jindrick does reveal the use of microprocessing technology with its type of control system, it has certain deficiencies or utilizes structure or methods which leave room for improvement in the art. The above-mentioned patents can be referred to regarding the basic subject matter of power transformers, tapchangers, and tapchanger controls.
Conventional state-of-the-art tapchangers are controlled by electromechanical or discrete electronic devices. They depend quite heavily on components which are subject to deterioration or change in operation. It is also to be understood that many tapchanger control systems require individual control hardware for each tapchanger of each transformer. Significant resources have to be expended to calibrate and maintain the mechanical and electronic controls over time for each transformer.
Quite simply, transformers and tapchanger controls age over time. As a result, accurate and reliable control of the transformers can become difficult. Damage to components may cause erratic and unreliable readings or operations. This in turn would most times require replacement of the entire system, at a very substantial cost.
Moreover, such structure can result in unreliable voltage regulation which in turn can cause customer dissatisfaction as well as result in costly expenditure of resources for maintenance.
A real need therefore exists in the art to provide a tapchanger control system which does not require complete replacement of existing tapchanger structure. The need exists for a control system which not only can accommodate changes over time in the performance of electrical and mechanical components, but also indicate when a failure or error occurs in one of the components.
Additionally, for power utilities, there is a need to maintain uniformity in the equipment that is used in each substation, as well as in all substations controlled by the company. While systems such as the Jindrick patent can be substituted for a substantial amount of tapchanger control structures in existing substations, a complete changeover would virtually be economically infeasible. Moreover, selective replacement would result in different transformers having different tapchanger controls. This makes it difficult to integrate the different type of controls, as well as maintain them. Different inventories of parts would be required, because repair must be complete and quick. Interruption of electrical service can be devastating to homes or businesses.
As previously discussed, the prime deficiency in electronic tapchanger controls like Jindrick is that they cannot control multiple transformers. More specifically, Jindrick requires an individual control for each transformer.
It is therefore a primary object of the present invention to provide a means and method for control of tapchangers for transformers which solves or overcomes the problems and deficiencies in the art.
Another object of the present invention is to provide a means and method as above described which allows flexible control of one or more tapchangers, whether the tapchangers are operated individually or in parallel.
Another object of the present invention is to provide a means and method as above described which utilizes a processor which can compare real time actual readings regarding performance of the transformer or transformers, and compare them to dynamic setpoints and ranges according to overall system operation.
A still further object of the present invention is to provide a means and method as above described which allows automatic adjustment of one or more transformer tapchangers according to variations in supply voltage or load.
A still further object of the present invention is to provide a means and method as above described which can detect malfunction or problems with the system.
A further object of the present invention is to provide a means and method as above described which presents a comprehensive system for reliably, efficiently, and economically operating transformer tapchanger controls.
Another object of the present invention is to provide a means and method as above described which efficiently minimizes circulating reactance vars in paralleled transformers, while at the same time maintaining regulation of voltage from the transformers.
Another object of the present invention is to provide a means and method as above described which eliminates a substantial amount of hardware required in conventional tapchanger controls.
Another object of the present invention is to provide a means and method as above described which eliminates imprecise and inefficient hunting for the correct voltage by tapchanger controls.
A still further object of the present invention is to provide a means and method as above described which allows tests and diagnostics to be accomplished during operation of the system, and provides fail safe mechanisms to insure the system is operating correctly.
Another object of the present invention is to provide a means and method as above described which reduces the amount of calibration and maintenance required for the tapchanger control system.
A still further object of the present invention is to provide a means and method as above described which is reliable, economical and durable.
These and other objects, features, and advantages of the present invention will become more apparent with reference to the accompanying specification and claims.